1. Field of the Invention
The present invention relates to an internal gear pump used as a lubricating oil pump for an engine or an electric pump for pumping hydraulic fluid in a transmission in an automobile.
2. Related Art
Conventionally, an internal gear pump such as a trochoid pump has been used as a pump, which pumps lubricating oil for an engine in an automobile. The internal gear pump includes an inner rotor that includes outer teeth, an outer rotor that includes inner teeth engaged with the outer teeth, pump housings within which the inner and outer rotors are received, a driving shaft that is attached to the inner rotor so as to be integrally rotatable therewith and protrudes from the pump housings to the outside, and a sprocket attached to a projection portion of the driving shaft which protrudes from the housings to the outside so as to be integrally rotatable therewith to transmit torque from the engine. The internal gear pump having the above-mentioned structure is configured such that the inner rotor is rotated when the driving shaft is driven by torque transmitted from the engine to the sprocket (for example, see JP-A-2003-286969).
In the internal gear pump in the above-mentioned related art, the driving shaft is driven by torque transmitted to the sprocket so that the inner rotor is rotated. However, the pump housings for receiving the inner and outer rotors therein and the sprockets for driving the inner rotor are juxtaposed in an axial direction. Therefore, there has been a problem in that the size of the internal gear pump in the axial direction thereof is relatively large.
On the other hand, the internal gear pump has been also used for as a pump for hydraulic fluid in a transmission in an automobile. In the transmission, the internal gear pump is mounted as an electric gear pump in which the internal gear pump is incorporated with an electric motor to drive the internal gear pump. Conventionally, such an electric gear pump includes an inner rotor that includes outer teeth and an outer rotor that includes inner teeth engaged with the outer teeth. One of the outer rotor and the inner rotor is driven to be rotated by the motor.
In the electric gear pump in such related art, since a pump part including inner and outer rotors and a motor for driving the pump part are usually juxtaposed in an axial direction, there have been many cases where the size of the electric gear pump is relatively large in the axial direction.
There has been a pump in another related art where an outer rotor to be driven is incorporated in a motor rotor and a gear pump is disposed within a motor so that the size of the pump in an axial direction is reduced (for example, see JP-A-2003-129966).
In the motor rotor in the above-mentioned related art, a sliding bearing is formed by providing a resin coat on an inner peripheral surface of the motor stator and an outer peripheral surface of the motor rotor, so that the motor rotor is rotatably supported. Since a small clearance is generally formed between the rotating part of the sliding bearing and the stationary part thereof, there is concern that small deviation of rotation axis occurs in the motor rotor, resulting in locking of the motor rotor and the motor stator to each other. When the motor rotor and the motor stator are locked to each other, it is not possible to obtain a stable driving torque and stable pump performance.
Further, according to the sliding bearing used in the related art, peripheral surfaces of the rotating and stationary parts come in contact with each other while the electric gear pump is stopped. Therefore, excessive friction may occur on the sliding bearing at the beginning of the rotation of the motor rotor. In addition, if the rotational speed is not appropriately increased during the rotation, an oil layer is not sufficiently formed between the rotating and stationary parts. For this reason, when the operation and stop of the pump are frequently repeated, there is a problem that abnormal wear may occur on the peripheral surfaces of the rotating and stationary parts due to the operation in addition to the above-mentioned axial deviation of the motor rotor, and the life of the electric gear pump is dramatically shortened.